Add method value(::Nothing)

[simsurace]

I did not notice this, but of course this is required to make the full use case work.
I threw in a nothing in the integration test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (master@ccfdc13). Click here to learn what that means.

❗ Current head d65911b differs from pull request most recent head a4a0e79. Consider uploading reports for the commit a4a0e79 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #68   +/-   ##
=========================================
  Coverage          ?   97.37%           
=========================================
  Files             ?        8           
  Lines             ?      229           
  Branches          ?        0           
=========================================
  Hits              ?      223           
  Misses            ?        6           
  Partials          ?        0           

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[willtebbutt]

Once CI passes, and the patch version has been bumped, I'm happy for this to be merged.